1. Field of the Invention and Contract Statement
This invention relates to an apparatus and method for displaying the approximate analog level of a variable electrical signal representing some physical quantity of interest.
2. Discussion of Background
Analog display devices are used extensively to indicate the levels of various physical quantities of interest, such as temperature, pressure, fuel consumption, vehicle speed, etc. The need to present this information in a limited space (for example, an automobile dashboard) often leads to a compromise between the use of relatively large, analog-type devices and smaller on-off devices. Analog devices, including moving-needle panel meters and bar-type graphic displays, have the advantage of showing where a voltage or other physical quantity lies within its operating range; simple on-off devices, sometimes called "idiot lights", require less space but can show only whether the quantity lies within or without its nominal range. For many applications, such as displaying automobile oil pressure or engine temperature, neither type of display may be wholly satisfactory: an "idiot light" may not provide sufficient information about the quantity of interest, but a larger analog display may not fit the space allotted or may not provide sufficient resolution.
Analog graphic displays are becoming increasingly popular as replacements for panel meters because they are relatively compact, easily read, have no moving parts, and provide more information than simple on-off devices. Such a display is formed by a matrix of light-emitting or light-reflecting segments, most commonly light-emitting diodes (LED's), and may take the form of a bar, circle, or fan display. An individual segment may be lighted or dark, depending upon the level of a variable input signal. In a "bar"-type display, the arrangement of light and dark segments forms a bar-like pattern whose length represents the input signal; alternatively, current is switched from one segment of the display to another in response to changes in the input, lighting (or extinguishing) one segment at a time to create the impression of a moving "dot".
Many methods exist for driving an analog graphic display, such as that typified by National Semiconductor Types 3914, 3915, and 3916 integrated-circuit display drivers where the input voltage is buffered and applied to a group of voltage comparators, each comparator fed with a different reference voltage. Depending on the relationship of the input voltage to each reference level, one or more LED's connected to the comparator outputs may be lighted. A control circuit allows either the "dot" or the "bar" mode to be effected. The reference voltages are provided by a resistor chain acting as a voltage divider. A wide variety of input signals and connection methods may be used. Different ratios between resistors in the chain provide either linear response (3914), logarithmic response (3915), or a logarithmic-like "VU" response which roughly matches that of the human ear (3916). All three versions are available either as separate DIP-packaged drivers (LM3914-6) or as preassembled graphic display modules (NSM3914-6) with built-in LED displays. Similar devices are available from many other manufacturers.
An analog graphic display has a typical resolution equal to the difference between successive reference voltages, that is, between the voltages needed to light successive LED's in the display. Application notes for the Type 3915 suggest a "smooth transition" method of improving the resolution by applying a triangle, sawtooth or sine wave in the frequency range 60 Hz-1 kHz to one end of the resistor chain; this has the effect of making each LED fade in gradually instead of turning on abruptly. Various other methods, such as an "exclamation point" display created by periodically shorting out the input signal to ground, are also suggested. Aside from these, there appear to be no commonly-accepted methods for improving the readability and resolution of these displays.
For a variable-frequency input signal, the effectiveness of the display depends on the relationship of the signal frequency to the flicker frequency, that frequency of a flashing light at which the persistence of human vision barely allows a visible flicker to be perceived. For frequencies above the flicker frequency, the light appears to glow steadily; below, it appears to flash. The flicker frequency varies with lighting conditions and from one individual to another, but is typically about 30 Hz. While the human eye can gauge the duty cycle--the relative proportion of "on" and "off" time within a single cycle--of a flashing light with reasonable accuracy, it is most sensitive when short flashes alternate with long "off" periods or when a continuous glow is broken by short interruptions. A change from 0% to 20% or from 80% to 1000% "on" time is easier to detect than a change from 40% to 60%.
FIGS. 1a-f show several cyclic waveforms which might be used to modulate a signal for display on a single-element or multi-element graphic display. For purposes of discussion, these waveforms are shown in idealized form. The time distribution function of a signal, or TDF, represents the fraction of time spent by an oscillating signal at each voltage within its range. For a function V=f(t) which has an inverse t=g(V), TDF is a function of V equal to the sum of absolute values of the first derivative of g(V), dg(V)/dV, at all points for which g(V) exists. It is usually normalized so that its integral over one complete cycle equals one unit. TDF represents the apparent brightness distribution of graphic display elements when operating in "dot" mode and displaying the function V=f(t)well above the flicker frequency.
The time distribution integral, or TDI, is a function of V defined as the integral of TDF downward from infinity to the corresponding value of V. It is defined as equal to one unit when V equals minus infinity, with values ranging from zero to one at practical values of V. It is a measure of the apparent brightness distribution of the display when operating in "bar" mode.
The Dirac delta function is a pulse of zero width, infinite amplitude, and finite area; when integrated it yields a step function proportional to this area. Appearing as an element of TDF, it represents that portion of the cycle time spent at a constant value.
TDF and TDI are shown for each waveform, drawn only approximately to scale.
FIG. 1a shows a constant input signal 10. Since the signal amplitude is the same at all times, TDF 12 is a Dirac delta function with unit area and TDI 14 shows a corresponding unit step. A single-element display driven by this signal would be the classic "idiot light", constantly on (or off) for input above some reference level and constantly off (or on) for input below the reference level, with no intermediate state. For a multi-element display in "dot" mode, this signal lights one element at a time at constant brightness, and the lighted area moves abruptly from one element to the next as the input level varies. The same is true in "bar" mode, except that several segments are lighted at the same time. A square wave (FIG. 1b) has alternating periods of constant amplitude 18 and 20, separated by sharp transitions 16. Because only two signal amplitude values are present, TDF 22 has two Dirac delta functions at corresponding levels, each with an area of one-half unit; TDI 24 shows two corresponding half-unit steps. This doubles the display resolution. For a multi-element display in "dot" mode, half-unit changes would create a display alternating between one full-brightness element and two adjacent elements either glowing at half-brightness, or, below the flicker frequency, flashing alternately with a 50% duty cycle.
FIG. 1c shows a sine wave 26. Since the signal amplitude varies continuously, TDF 28 is also continuous. Some sine-wave interference is always present in real-world signals, so every practical display has some modulation of this type. TDI 30 resembles that of a square wave, but with rounded corners. This type of modulation produces output with a duty cycle rapidly rising to about 50%, remaining near that value through a large part of its range, then rapidly increasing again to 100%. Transitions between two adjacent display elements have some degree of overlap during which both segments are lighted.
FIG. 1d shows a triangle wave 32. This gives a further increase in display resolution over the square and sine waves, as it allows small changes at intermediate values of V to have the same effect on the duty cycle as equivalent changes at high or low values. Since the signal spends an equal amount of time at each voltage, TDF 34 is flat between the two voltage extremes and TDI 36 changes steadily during a cycle.
FIG. 1e shows a sawtooth wave 38, identical with the triangle wave except for having a portion 40 time-reversed, forming portion 42. TDF 44 and TDI 46 have the same appearance as those of the triangle wave. This results from the fact that TDF and TDI are invariant with time-reversal of any part or parts of the waveform.
When an input signal is modulated at frequencies above the flicker frequency, the corresponding response with any of the above-described waveforms is partial-intensity lighting of an LED within the analog range, as in Ogita's U.S. Pat. No. 4,348,666 issued Sept. 7, 1982. For example, with square-wave modulation (FIG. 1b) the intensity is constant at 50% throughout the analog range. Other types of modulation produce varying intensities which reflect to some degree the position of the input level within its range.
To accommodate the sensitivity of human vision, changes at intermediate signal amplitudes should have larger effects on the duty cycle than those at high and low values: as noted above, small duty-cycle changes are more easily perceived near the extremes of the range than at mid-range. A disadvantage of these waveforms is that they do not match this characteristic of human vision: the sine wave favors intermediate duty cycles at the expense of high and low ones, while the triangle or sawtooth waves show no preference.
A mathematical function which does have this property is the tangent 48 (FIG. 1f). Over a half cycle, the tangent begins at minus infinity, rises with ever-decreasing slope to zero, then continues increasing to plus infinity; it then undergoes an abrupt, infinitely-fast downward transition 50 before beginning the next cycle. TDF 52 resembles a Gaussian function, with a central peak flanked by gradually-decreasing "tails"; TDI 54 resembles the Gaussian integral. However, this function cannot be realized in a practical circuit due to its infinite peak amplitude and zero transition time.